A gas turbine engine includes one or more rotor shafts which are suitably supported by annular frames. In one exemplary engine, a fan is joined to a fan shaft which is supported at its forward end in a fan frame. The fan frame includes an outer casing, an inner hub, and a plurality of circumferentially spaced apart struts extending therebetween. The hub supports a bearing for the rotating fan shaft, with the loads therefrom being channeled through the hub and struts into the outer casing. The struts are airfoil shaped since inlet air to the engine first passes between the adjacent struts prior to reaching the fan.
The struts are typically hollow members for reducing weight of the engine while also providing passages through which conduits may be disposed for carrying air or oil, for example, through the struts. Since the struts are hollow and support the rotating fan shaft and allow airflow between adjacent struts, they are subject to excitation forces from the rotating fan shaft and from the air which may cause vibration thereof. Where the natural frequencies of the frame approach the excitation frequencies from the rotating fan, conventionally known as (per)/rev frequencies, and from the inlet airflow, fundamental flexural and torsional modes of vibration, as well as panel modes of vibration may be excited in the individual struts.
It is conventional to either design the frame to provide a suitable margin between the excitation frequencies and the natural resonance frequencies of the frame, or to provide suitable damping material for damping any vibrations which may result. For example, the hollow passages through the struts may be conventionally filled with a damping compound such as rubber for damping vibration of the struts themselves as well as damping vibration of the conduits therein. However, improved frame damping is desired especially where the frames are being made lighter in weight which decreases the margin between the excitation forces and the natural resonance frequencies thereof.